We are interested in determining how the initiator site of mRNA is selected by the ribosomes in bacterial protein synthesis. We intend to study the problem with E. coli ribosomes by kinetic analysis of the binding of fMet-tRNA and of fMet-puromycin formation, using limiting concentrations of natural and synthetic nRNAs. The reaction with limiting mRNA is pseudo-first order with respect to mRNA concentration, and the rate constants should therefore reflect directly the efficiency of the mRNAs. If the rate constants with synthetic RNAs, which are unlikely to be specifically recognized by the ribosome and/or initiation factors, are comparable to or greater than the rate constants of natural mRNAs, the results would imply that no special initiation signal in mRNA is obligatorily recognized in initiator site selection. As secondary objective, we will study the basis for the difference in the translational specificity of E. coli and B. stearothermophilus ribosomes and for the activation of E. coli ribosomes by the supernatant factors EF-Ts and EF-G. Additionally, we will examine the specificity of rabbit reticulocyte ribosomes and initiation factors, as a preliminary step to the investigation of the regulation of eukaryotic protein synthesis, by kinetic analysis similar to that described above.